Abstract

The ability of some insects to engage in complex orchestrations of tracheal gas exchange has been well demonstrated, but its evolutionary origin remains obscure. According to a recently proposed hypothesis, insects may employ spiracular control of gas exchange to guard tissues against long-term oxidative damage by using the discontinuous gas-exchange cycle (DGC) to limit internal oxygen partial pressure (P_(O_2)). This manuscript describes a different approach to oxygen guarding in the lower termite Zootermopsis nevadensis. These insects do not display a DGC but respond to elevated oxygen concentrations by restricting spiracular area, resulting in a transient decline in CO_2 emission. High internal CO_2 concentrations are then maintained; restoring normoxia results in a transient reciprocal increase in CO_2 emission caused by release of excess endotracheal CO_2. These changes in spiracular area reflect active guarding of low internal O_2 concentrations and demonstrate that regulation of endotracheal hypoxia takes physiological priority over prevention of CO_2 build-up. This adaptation may reflect the need to protect oxygen-sensitive symbionts (or, gut bug guarding). Termites may eschew the DGC because periodic flushing of the tracheal system with air may harm the obligate anaerobes upon which the lower termites depend for survival on their native diet of chewed wood.